With the development of communication technologies, a mobile communication system has been evolved into a System Architecture Evolution (SAE) system. The SAE system includes an Evolved Universal Terrestrial Radio Access Network (E-UTRAN), and the E-UTRAN includes a core network and a wireless access network.
FIG. 1 is schematic diagram illustrating an E-UTRAN in a SAE system in the prior art. As shown in FIG. 1, an evolved Node B (eNB) belongs to the wireless access network, and the core network includes a Mobility Management Entity (MME) and a Subscriber Gateway (S-GW). The eNB is adapted to provide a wireless interface for User Equipment (UE) such as a handset and so on. The MME is adapted to manage mobility contexts and session contexts of the UE, and issue information related to security to a user through packets. The S-GW is adapted to provide functions of a subscriber plane. The MME and the S-GW may be in the same physical entity. Generally, the S-GW transmits user data streams through a GPRS Tunneling Protocol (GTP) to the eNB to which the UE belongs, and then the eNB transmits the user data streams to the UE. Each eNB is connected with multiple MMEs in an MME pool, and also is connected with multiple S-GWs in an S-GW pool. An interface between eNBs is called as an X2 interface, and an interface between the eNB and the MME or between the eNB and the S-GW is called as an S1 interface.
In order to effectively utilize air interface resources, some mobile communication services are provided to users in a broadcast and multicast mode, and these mobile communication services are called as Multimedia Broadcast and Multicast Services (MBMS). Each MBMS is provided in its own serving area, and in each cell of the serving area, a special control channel is used to transmit MBMS signallings. A Broadcast Multicast Service Center (BM-SC) is a multimedia broadcast and multicast service providing center, MBMS data are transmitted from the BM-SC to the S-GW in a SAE network, then are transmitted to a corresponding eNB by the S-GW, and finally are transmitted to a user by the eNB.
The MBMS service may be transmitted in a single carrier cell, and id different cells use different carriers, a user on the boundary of a cell only receives the MBMS service of the current cell. If adjacent cells use the same carrier to transmit the same MBMS service, and transmit the MBMS service in a synchronization mode, the user on the boundaries of the adjacent cells can receive a signal obtained by overlapping energies of the two MBMS services. Therefore, in the prior art, a continuous area is defined. In this area, all eNBs use the same carrier to synchronously transmit the same MBMS signal, to improve the receiving quality of the user's MBMS service. The continuous area is called as a Single Frequency Network (SFN) area.
The SFN area includes a group of cells which are continuous geographically, and the cells use the same radio resources to synchronously transmit a specific MBMS service. The SFN area exclusively belongs to one MBMS serving area.
The performance can be improved obviously through synchronously transmitting MBMS data by all eNBs in the SFN area. At present, different technologies may be used to implement the data synchronization transmission between eNBs. In one technology, a network provides synchronization, i.e. a transmission network obtains synchronization through clocks. In this technology, an IEEE1588 protocol may be used, to coordinate clocks of all eNBs and make these clocks synchronous, and the accuracy of these clocks is in a microsecond level. In another technology, a synchronization signal is transmitted to each eNB through a common satellite signal, e.g. a Global Position System (GPS). No matter which technology is used, the object is to make all eNBs synchronously transmit signals, to implement the overlapping of signals of different eNBs, thereby improving signal quality.
When MBMS data are transmitted from the BM-SC to each eNB, it is possible that data packets are lost or lagged, so it is needed to provide some mechanisms to guarantee that data transmitted by all eNBs can keep synchronous even if the data packets are lost or lagged.
A conventional mode includes that the BM-SC adds a synchronization frame head into a MBMS data packet, and the synchronization frame head mainly includes a time stamp and two counters. The time stamp is used to indicate absolution time in one synchronization period, one counter is used to indicate an Elapsed Octet Counter transmitted in one synchronization period, and the other counter is used to indicate a Packet Number in one synchronization period. The two counters are set as 0 at the beginning of each synchronization period.
MBMS data packets may be lost when being transmitted from the BM-SC to the eNB, and the eNB determines how many data packets and bytes are lost during the transmission procedure according to the Elapsed Octet Counter and the Packet Number in the synchronization frame head. The eNB needs to determine how many radio resources are occupied by the lost data packets, and fills filling bits of which quantity is the same as the quantity of the lost data packets on the radio resources. A simple example is taken hereinafter to describe how the eNB calculates the lost data.
For example, the eNB1 has received a first packet, the Packet Number of the first packet is equal to 1, and the Elapsed Octet Counter of the first packet is 50 bytes; the eNB1 receives a second packet again, the Packet Number of the second packet is 3, and the Elapsed Octet Counter of the second packet is 100 bytes; the eNB may determine that one packet is lost, the Packet Number of the lost packet is 2, and the Elapsed Octet Counter of the lost packet is 50 bytes; and thus the eNB1 needs to reserve air resources of 50 bytes on which any data can not be transmitted or predefined filling bits are transmitted. Another eNBx receiving the packet of which Packet Number is 2 transmits the packet normally. In this way, it can be guaranteed that the eNB1 and the eNBx can transmit the packet of which Packet Number is 3 by using the same air resources.
The time stamp is used to indicate absolution time, to tell the eNB when data are transmitted. According to a definition, the absolution time indicated by the time stamp is multiples of 10 ms.
In the prior art, time stamps of data packets of the same service are identical in each synchronization period of the BM-SC, and are calculated according to time at which the BM-SC receives the first data packet of the service in the synchronization period, a calculating formula is as follows:time stamp=time at which the BM-SC receives the first data packet+the length of the synchronization period+a delay.
The delay relates to the largest transmission delay and a processing time. The length of the synchronization period and the delay are configured by an operating and maintaining system. The change of the time stamp means the start of a new synchronization period.
After receiving a data packet, the eNB determines, according to the time stamp, the time at which the data packet is transmitted. The eNB caches, multiplexes and transmits data in one transmission period, and this time period is called as a MCH Subframe Allocation Pattern (MSAP) time period. Generally, the length of the MSAP time period is the same as the length of the synchronization period on the BM-SC, but on the eNB, the time period is called as the MSAP time period or a transmission period. The MSAP time period corresponds to certain physical resources. The physical resources are composed of subframes with a certain format. The subframes do not need to be continuous in time, and the format is called as MSAP. The eNB provides dispatching information to the UE, and the dispatching information relates to one MSAP time period. The dispatching information is used to tell the UE which MBMS service is received on which subframe.
FIG. 2 is a schematic diagram illustrating an implementation in which data packets of the same service are transmitted in the same synchronization period in the prior art. Suppose that the length of a synchronization period is 60 ms, the delay is 10 ms, and at 10:00:00:000, the BM-SC receives the first data packet of a service 1, called as a data packet 1 of the service 1, a time stamp added to the data packet 1 by the BM-SC is 10:00:00:000 (hour:minute:second:microsecond)+the length of the synchronization period+the delay=10:00:00:000+70 ms. After 10 ms, the BM-SC receives the second data packet of the service 1, called as a data packet 2 of the service 1, and the same time stamp of 10:00:00:000+70 ms is added to the data packet 2 because the data packet 2 and the data packet 1 are transmitted in the same synchronization period. The eNB has its own clock, and air resources corresponding to 10:00:00:000+70 ms are a radio time slot 1, so the data packet 1 of the service 1 should be transmitted at the time corresponding to the radio time slot 1, and the data packet 2 of the service 1 should be transmitted following the data packet 1.
FIG. 3 is a schematic diagram illustrating an implementation in which data packets of different services are transmitted in the same synchronization period in the prior art. Suppose that the length of a synchronization period is 60 ms, the delay is 10 ms, and at 10:00:00:000, the BM-SC receives a data packet 1 of a service 1, a time stamp added to the data packet 1 by the BM-SC is 10:00:00:000+70 ms. After 10 ms, the BM-SC receives a data packet 1 of a service 2, and a time stamp added to the data packet 1 of the service 2 is 10:00:00:010+70 ms=10:00:00:000+80 ms. For the eNB, air resources corresponding to 10:00:00:000+70 ms are a radio time slot 1, so the data packet 1 of the service 1 should be transmitted at the time corresponding to the radio time slot 1; air resources corresponding to 10:00:00:000+80 ms are a radio time slot 2, so the data packet 1 of the service 2 should be transmitted at the time corresponding to the radio time slot 2. If the data packet 1 of the service 1 is very large and the transmission of the data packet 1 is not be completed in the radio time slot 1, the data packet 1 of the service 2 can not be transmitted in the radio time slot 2 and needs to be delayed. Vice verse, as shown in FIG. 4, if the data packet 1 of the service 1 is very small and only a small amount of data are transmitted in the radio time slot 1, the air resources can not be fully utilized, which results in waste. FIG. 4 is a schematic diagram illustrating another implementation in which data packets of different services are transmitted in the same synchronization period in the prior art.
Sum up, in the conventional data synchronization method, air resources can not be fully utilized.